Are Earth’s magnetic reversals becoming more frequent?

New findings reported in Nature Geoscience suggest that the Earth's magnetic …

The Earth's magnetic field does a lot for us and the rest of Earth's denizens. First and foremost, it helps us and some birds figure out which direction is north. Second, and in some ways more important in today's wired society, it shields the Earth from charged particles coming from the sun, an effect that was recently used to put to rest one of the claims that solar activity is producing current climatic warming trends. As a result, changes in Earth's magnetic field can have important consequences.

The Earth's magnetic field is set up by a dynamo created by the rotation of the outer core. Over long periods of time, the chaotic motions of the core may occasionally lead to a weaker field which can collapse. When this field reappears it may be reversed. However, even when the field collapses, it is likely to re-form with the same polarity because of influence from the inner core. The inner core is solid, and can only change its magnetic orientation very slowly.

Because of this interaction between the inner and outer core, one might expect geomagnetic reversals to be more common when the inner core was smaller, billions of years ago. Not so, says a recent Nature Geoscience paper. The researchers studied the magnetic orientation of rocks that formed 2.82 to 2.45 billion years ago. Although "records from that era are spotty at best," researchers managed to dig up1 data for some fast cooling igneous rocks that covers 130 million years of time spread out over that 380 million year period. Their findings indicate that contrary to previous studies, the Earth's magnetic field reversed polarity less frequently 2.5 billion years ago than it does today.

While they are looking at a very limited time window, these results are consistent with previous modeling results. A previous study modeled the geodynamo using three different sizes of inner core: one a quarter the size of the present core, one the same size as present, and one twice the size of the present one. They found that the simulation with a smaller core (representative of the Earth billions of years ago) resulted in a more stable magnetic field than the present or future cores. They attribute this change to the resulting thickness of the outer core. A thinner outer core (larger inner core) will enable different portions of the core to act with greater independence, while a thicker outer core (smaller inner core) will be more tightly integrated by mixing processes.

The authors finish by noting that the limited amount of data do not permit any firm conclusions, but their data are consistent with a magnetic field that is becoming less stable over time. This does not mean that we are going to have a reversal any minute now, although separate measurements of the geomagnetic field suggest that it is weakening and may reverse sometime in the next three to four thousand years.